Improved heat transfer tube technology in recent years has been highly dependent upon the improvement of two phase heat transfer, that is the transfer of thermal energy due to the phase transformation from the liquid to the vapor phase. The methods to improve this two phase heat transfer include both passive and active techniques. Passive techniques include surface treatments, roughening the surface, extending the surfaces, displaced enhancement, swirl flow techniques, alteration of surface tension, and the inclusion of additives to the coolant. Active techniques include mechanical aids, surface vibration, fluid vibration, and the addition of electrostatic fields.
In the area of treated surfaces, various materials are deposited on the heat transfer tube surfaces to promote boiling. Such materials have included Teflon, tube surface oxides, and the addition of high surface copper powder. These surface treatments improve the wettability of the surface and result in a low wall super heat which eliminates boiling curve hysteresis.
Surface roughening is a technique to provide a large number of nucleation sites on the tube surface. The technique involves the mechanical deformation of the surface to provide a large number of reentrant cavities.
Extended surface tubes are produces by finning techniques which yield high external surface areas to the tube and allow very large heat transfer rates if the base temperature is in the film boiling range; however, nucleate boiling is not promoted with this type of heat transfer tube.
Displaced enhancement techniques promote boiling by taking advantage of hydrodynamic instability in the coolant when open structures are placed directly above the heat transfer surface.
Surface tension devices operate on the wicking principle which relies on capillary forces while the addition of additives to the coolant affects the wettability of the coolant to the heat transfer tube.
A number of mechanical boiling aids have been proposed including rotating of the boilers themselves, the introduction of rotating plates, and the introduction of bubbles into the vicinity of the heat surface.
The purpose of vibrating either the fluid or the surface is to form localized nucleate boiling sites due to pressure variations in the liquid. The use of electrostatic fields improves mixing within the coolant and is used principally with poorly conducting or dielectric fluids.
Of the above techniques, those that promote nucleate boiling are of principal interest from a technological viewpoint. The theory of nucleate boiling has been well developed and is well understood at this point. The variables that are involved in promoting vapor phase nucleation are well understood. The parameters of importance in a nucleate boiling tube-coolant system include the specific heat of the liquid, the specific heat of the tube material, the heat transfer coefficient, the latent heat of vaporization, the thermal conductivity of the liquid and the heater tube, the geometry of the nucleation site, the temperature of the coolant, vapor, and surface, the liquid viscosity, the surface tension, and the densities of the liquid and vapor phases.
The nucleate boiling phenomenon involves two separate operations. The first of these is the nucleation of the vapor phase within the liquid while the second is the subsequent growth of the vapor phase to form bubbles within the liquid. It has been postulated that improved efficiency of heat transfer can be attained when the nucleation process does not have to be continuously redone. This nucleation process requires a large amount of superheating. Improved efficiency can be observed if the thermal energy is transferred by the growth of pre-existing vapor phase nuclei. This approach has resulted in the specification of re-entrant cavities as highly effective nucleate boiling sites.
A number of patents have been issued whereby the surface of a heat transfer tube is mechanically altered to provide these re-entrant sites. These include U.S. Pat. No. 3,326,283 by Ware, U.S. Pat. No. 3,454,081 by Kun, U.S. Pat. No. 3,566,514 by Szumigala, U.S. Pat. No. 3,881,342 by Thorne, and U.S. Pat. No. 3,906,604 by Kakizaki. While all of the above patents propose the improvement of nucleation by the mechanical introduction of nucleation sites, they all suffer from the common characteristic of having a relatively few number of nucleation sites per given area of tubing surface. This limitation is imposed by the manufacturing tooling required to produce the tubes, and is an inherent limitation for any mechanically produced tube.
The demonstrated heat transfer capability of a tube produced with a much higher density of nucleation sites is covered in U.S. Pat. No. 3,384,154 by Milton. This tube is of the treated surface variety mentioned above where copper powder particles are sintered to the surface of the heat exchanger tube. This provides a very high density of nucleation sites on the tube surface and allows retention of the vapor phase throughout the open pore structure of the sintered surface.
This sintered surface tube, while an effective boiling surface and heat transfer tube, suffers from manufacturing difficulties. The copper powder is mixed with an organic binder and sprayed onto the tube surface for ease of handling. The coated tube is then subjected to a high temperature exposure. This decomposes the organic binder and sinters the copper particles together as well as to the base tube. The Milton patent states the sintering temperature to be about 1760.degree. F. which is about 180.degree. F. below the melting point of copper. This high temperature treatment is not only difficult to do but can result in serious degradation of the mechanical properties of the base tube. The degradation problems can be minimized by utilizing alloys whose superior recrystallization and grain growth characteristics will reduce the amount of property degradation but such alloys introduce added cost and have lower thermal conductivity.